A human elbow permits pivoting movement of an ulnar bone relative to the connected humeral bone. Should the elbow joint become damaged by disease, accident, or the like, then the ulnar and humeral bones may not have an acceptable level of relative pivotal motion. Although artificial elbows have been proposed, none achieves the range of mobility which approaches that which is naturally occurring.
For replacement of an elbow to occur, then there is a need to provide a connection to the ulnar bone and the humeral bone. One means for connecting the bones is by implanting into each an artificial member which extends into the shaft or medullary canal thereof. These members may be cemented within the canal, with their adjacent free ends interconnected by some sort of hinge mechanism. A particular disadvantage of a single hinge artificial elbow is due to the transfer of force from the hinge into the bone cement interface of the humeral or ulnar member. Eventually the humeral or ulnar member may become loosened within its medullary canal by this unrestricted or unmitigated force transfer. The longevity of single hinge implants subjected to these high stresses has been unsatisfactory, and replacement may be required.
Prior application Ser. No. 07/740,918, the disclosure of which is incorporated herein by reference, is directed to an implantable artificial elbow wherein two hinge points are provided by a connecting link which is pivotally connected to the free ends of the ulnar and humeral members. The double hinge connection minimizes the transfer of force into both medullary canals of the humeral and ulnar bones, with the result that implantation longevity is greatly increased. Furthermore, because of the double hinge mechanism, which to a certain extent resembles a double hinge nut cracker, the ulnar and humeral bones can be pivoted to a greater extent relative to each other. The connecting link provides a reservoir or space within which the muscle mass is positioned.
In addition to pivotal motion of the ulnar relative to the humeral bone, the human elbow permits the ulnar to move sideways transverse to its pivot axis. No artificial elbow known to us has this capability.
Another disadvantage of prior artificial elbows is the use of metal components, which wear on account of the single pivoting motion. Should the components become worn, then the patient will eventually lose motion with the prosthesis and may require a replacement. Because the components have worn, then the load bearing surfaces must be replaced in their entirety by another surgical procedure.
The disclosed invention is an artificial elbow which has a dual hinge pivoting mechanism, and the ulnar member is maintained in a semi-constrained state while the humeral member is maintained more rigidly restricted and constrained so that a more natural motion may occur without instability. Furthermore, sideways motion of the ulnar is permitted because of the manner in which the ulnar member is attached to the connecting segment. In addition, polymeric materials are provided at contact points, in order to minimize wear of metal components. The polymeric components are easily replaced so that the ulnar and humeral members may be left in place when worn components are replaced. The differential torque requirements of the two pivots restricts the simultaneous joint pivot motion that could result in joint instability.